Magnetic resonance imaging (MRI) is one of the major imaging techniques in medicine. MRI is capable of generating detailed images of soft tissues. In MRI, specific properties of the various compounds found inside tissues are used to generate images, e.g., water is most commonly used for this purpose. When subjected to a strong external magnetic field, the protons 1H will align with this external field, resulting in a magnetic moment. After excitation by radio frequency RF pulses, this magnetization will generate an RF signal that can be detected. This RF signal is characterized by a frequency that is related to the magnetic field strength. Therefore, magnetic field gradients are used to encode a spatial information which is needed to reconstruct the image from detected signals.
One prerequisite for performing MRI in a highly accurate manner is the exact knowledge of local magnetic field inhomogeneities within the examination volume, the sensitivities of the used coils to detect RF signals generated by the above mentioned magnetization, as well as to have detailed information about coil timings or in other words to know how quickly a coil is able to detect RF signals at highest accuracy after having been turned on.
Knowing the MR field inhomogeneities allows to largely correct them, e.g. by application of higher-order shimming. Homogeneity of the magnetic field can be important for a variety of reasons. One of them is spectroscopy, but also in MR imaging (the primary focus of our attention here) it is important for some sequences, e.g., balanced field-echo techniques (also abbreviated as bFFE, a.k.a. true-FISP a.k.a. FIESTA), but also the separation between water and fat—most notably for the techniques of fat suppression.
The reason why a coil sensitivity with respect to an image point of an examination volume has to be known is that, for example coils have the property of decreasing reception sensitivity with increasing distance from the magnetic resonance signal source, wherein said sensitivity distribution is not uniform over the whole imaged examination volume. This is an extremely important issue especially in the case, where multiple reception coils record signals from the same image point in an examination volume. In this case, received signals from said multiple reception coils have to be weighted in such a manner, that a uniform magnetic resonance image is obtained using the information from all coils.
For example, WO 2006/018780 A1 discloses a magnetic resonance (MR) method for a quantitative determination of local relaxation time values in an examination volume. Thereby, local magnetic field inhomogeneity values are calculated from local resonance frequency values measured with a plurality of echo signals.
EP 0545465 A1 discloses a fast and simple shimming method which utilizes only one measuring sequence.
U.S. Pat. No. 6,275,038 B1 discloses a method for evaluating an inhomogeneity in a magnetic polarizing field used to acquire an MRI image of a slice of a subject at a point in the slice. Thereby, by using phase differences between values of a first and second spatial image at said point, magnetic field inhomogeneities are evaluated.
U.S. Pat. No. 7,015,696 B2 discloses a method for calculating a sensitivity distribution of a receive coil.